Process for separating a mixture of isomeric xylenes



H. P. HETZNER El' AL PROCESS FOR SEPARATING A MIXTURE OF ISOMERIC XYLENES June 13, 1.950

2 Sheets-Sheet l Filed June 20, 1946 SL *l w fww w. Q J SESS Wg QN lNVENTORS Howard P /,o/zner ATTORNEY June 13, 1950 H. P. HETZNER ET Al.

PRocEss FOR SEPARATING A MIXTURE oF IsoMERIc xYLENEs 2 Sheets-Sheet 2 Filed June 20, 1946 B'IGVHBAODBH VHVd ATTORNEY Patented June 13, 1950 PROCESS FOR SEPARATING A MIXTUBE 0F ISOMERIC XYLENES Howard P. Hetzner, San Anselmo, and Robert J.

Miller, Berkeley, Calif.,

Research Corporation, corporation of Delaware assignors to California San Francisco, Calif., a

Application June 20, 1946, Serial No. 678,142 7 Claims. (C1. 26o-874) This invention relates to the preparation or recovery of isomeric dialkyl benzenes from complex hydrocarbon mixtures. More particularly, it involves the production of separate ortho, metaand para-xylene fractions from nonaro matic petroleum sources in relatively high yields and purity.

Although other sources of hydrocarbons may be utilized, the invention will be here described and illustrated for purposes of simplicity in connection with petroleum hydrocarbon mixtures. In practicing the invention in its preferred embodiment, suitable hydrocarbon fractions for the production of the separate xylene isomers are obtained by aromatization. preferably utilizing the so-called hydroforming process. This type of process is well known in the petroleum industry. However, because the chemistry involved y and the mixtures obtained are extremely complex, careful coordination of feed stocks and hydroforming conditions is required in order to obtain best resultsand to yield a suitable xylene fraction for isomer recovery.

The present invention is particularly adapted to the treatment of an equilibrium xylene mixture from hydroformed non-aromatic petroleum fractions. The term equilibrium xylene mixture is here utilized to designate a xylene fraction containing ortho, metaand para-xylenes in the equilibrium proportions resulting from a hydroforming or other suitable aromatization process. Although the invention is particularly adapted to the treatment of this specic type of mixture, it will be understood by those skilled in the art that the invention is also applicable to other xylene fractions from other suitable sources and particularly to those xylene mixtures which are no more complex than the equilibrium xylene fractions hereinafter described. To avoid prolixitythe remainder of this description will be made with reference to xylene fractions de rived from hydroforming operations with the intention that the invention shall not be limited thereby.

The isomeric xylenes diifer, of course, only in that the two methyl groups in the benzene ring are in the ortho, meta and para relative positions respectively. The differences in chemical and physical properties of these position isomers have been insuiiicient to afford adequate separation and recovery thereof in good yields and purity. Although prior art methods have per- 2 mitted separation of one isomer, such separation has, so far as known to us, been at the expense of high loss or inadequate purity of one or more of the remaining isomers. That is to say, in the prior art processes recovery of one isomer has been obtained but without reference to saving or recovering each of the remaining isomers separately. These prior deficiencies and problems become more acute when production of xylene isomers from non-aromatic petroleum fractions is contemplated. This follows from the fact that petroleum contains, or even consists of, non-aromatic hydrocarbons.

An object of this invention is to provide an improved method of recovering each of the xylene isomers from mixtures thereof with each other or with other hydrocarbons.

Another object .of the invention is to provide a process wherein a xylene fraction is successively conditioned for recovery of each isomer by new combination of process steps, whereby improved recovery of each isomer fraction in acceptable purity is obtained.

A further object is to provide a process for obtaining separate ortho, metaand para-:wlene fractions from a hydroformed petroleum mixture.

Additionally, an object of the invention is to furnish a process for separating an equilibrium mixture of isomeric xylenes into an ortho-xylene fraction, a. meta-xylene fraction and a paraxylene fraction, while avoiding substantial sacriilce in yield of any xylene isomer.

Another object is to provide an improved process for recovering para-xylene and/or producing the same from non-aromatic petroleum distillates.

Further objects and advantages of the invention will be apparent from the following description and drawings in which Figure l is a diagrammatic owsheet of the principal process steps herein utilized and Figure 2 is a chart illustrating the effect of the presence of ortho-xylene and meta-xylene on the amount of para-xylene which can be recovered from a mixture of xylenes.

Briefly described, a, process embodying the principles of this invention comprises the steps (hydrov recovery of a meta-xylene fraction while simultaneously separating an ortho-xylene fraction. by superfractionation to form a lower-boiling overhead distillate conditioned for selective removal of meta-xylene and to separate a higherboiling ortho-xylene fraction;

(3) Conditioning said lower-boiling overhead distillate for separation of a para-xylene fraction while simultaneously recovering a metaxylene fraction by (a) Partially sulfonating said overhead distillate to form an unsulfonated hydrocarbon layer conditioned for separation of paraxylene and to selectively form meta-xylene monosulfonic acid,

(b) Separating the meta-xylene sulfonic acid from the unsuli'onated hydrocarbon mixture and recovering meta-xylene by selective hydrolysis of said separated sulfonic acids;

(4) Separating a para-xylene fraction from said conditioned unsulfonated hydrocarbon mixture by (a) selectively solidifying a, para-Xylene fraction in a low-temperature cooling stage and separating said solidified fraction from unsolidiled hydrocarbon mother liquor,

(b) Melting said solidified para-xylene fraction and again selectively solidifying a para-xylene fraction from said melted hydrocarbon mixture in a higher-temperature oooling stage;

5) Recovering said last selectively solidined para-xylene fraction.

The aromatization process and the feed stocks therefor are only supplemental to this invention, but an adequate understanding of the selection of feed stocks and control of the aromatization process to yield a proper xylene cut is essential to the most successful practice of this invention.

`Feed stocks Naphthenic hydrocarbon mixtures from naphthene-type petroleum crude oils comprise one preferred type of feed stock. Such mixtures are normally termed straight run distillates in the 50 and dehydrogenation petroleum industry. The hydrocarbons present in this preferred feed stock are believed to consist largely of cycloaliphatic hydrocarbons with six carbon atoms in the cycloaliphatic ring and with aliphatic side chains attached to the ring. Some by isomerization and five and seven carbon atom cycloaliphatic rings may be present. Both the number of side chains and the length of each chain attached to the foregoing rings vary among the many compounds normally present in a petroleum hydrocarbon mixture. In general, these variables are a function of the average molecular weight or, more particularly, the boiling range and distillation curve of the petroleum fraction. A naphthenic hydrocarbon mixture consisting essentially of hydrocarbons having from six to twelve carbon atoms in the molecule and preferably composed at least predominantly of hydrocarbons containing from seven to eight carbon atoms at present is regarded as 'a more desirable feed stock. 'Ihe fraction selected desirably should boil within the range of from about 180 F. to about 420 F., and preferablyirom about 180 F. to about 320 F. In some instances an even more narrow cut boiling from 230 F. to 275 F. is preferred.

4 Aromatization As previously set forth. an initial step in the exemplary process comprises aromatization of the particular petroleum feed stock selected. Where a naphthenic hydrocarbon mixture is utilized, the conversion of hydrocarbons to aromatics isA believed to occur by dehydrogenation of the sixcarbon atom rings from cycloaliphatic to aromatic while leaving alkyl groups attached to the residual nucleus. For example:

mo ori--ornv om BHI BiCA OZB-OE: OH;

0H: n onl HxC CHI 3H: BiC CH-CH 0H;

CH: H CH:

H10 CH:

3H: HIC CHI H Ha El CHICK: E CHICK: v H|C CHI -l- 3H| HIC CH:

Isomerization of any C1 alicyclic rings present to aromatic compounds also Likewise, Cs alicyclic rings are converted to aromatics dehydrogenation. These various reactions represent an over-simplification of the aromatization reactions which may actually. occur, since de-alkylation and shortening is believed to occur. containing side chains of side chains as by cracking undoubtedly take place. In any event, the aromatization reaction product comprises a highly complex mixture of aromatics and also contains non-aromatic hydrocarbons such as saturated or unsaturated paraillns and naphthenes. The overall complexity of the mixture and the relative proportion of the above-mentioned non-aromatic components depend upon the effectiveness of the particular aromatization process as well as upon the specic hydrocarbon feed stock selected. It is for this reason that a highly naphthenic hydrocarbon feed stock boiling within the ranges previously disclosed are preferred. since the reaction products therefrom are better adapted to subsequent processing steps involved in the production of isomeric xylenes. However,v it is possible. but less desirable. to obtain operative aromatic fractions i'rom paramnic hydrocarbons by known reaction, such as dehydrogenation and cyclization illustrated by the following reactions:

Processes for effecting such reactions and catalysts therefor are known in the petroleum art. Likewise, aliphatic oleflns are convertible to aromatics by known cyclization and dehydrogenation reactions similar to the foregoing. These various known processes may be utilized within the broader aspects of this invention and are embraced within the term aromatization as used in the present specification.

The preferred aromatlzation process known as hydroforming is characterized by aromatization in the presence of controlled amounts of going residual liquid to leave a hydrocarbon mixture boiling above about 275 F.;

(3) Separating from the above last heavier hydrocarbon mixture a xylene fraction boiling within the range of from about 275 F. to about 295 F.;

(4) Reducing,V if necessary. the non-aromatic hydrocarbon content of the foregoing 275 F. to 295 F. xylene fraction preferably to less than about by weight, and more desirably to a negligible value equaling or approaching 0.

In those instances where the aromatization process is suitably carried out and the 275 F. to 295 F. xylene fraction is substantially 100% aromatic, this fourth step will become unnecessary.

. However, when and if a substantial non-aromatic lwdrocarbon content is present, these undesired hydrocarbons may be eliminated; for example, by a selective solvent distillation method, such as described in Cope et al. Patent No. 2,215,915, issued September 24, 1940.

The foregoing distillation and purification steps are hereafter referred to as fractionation and yield a xylene cut more particularly dehydrogen and a vanadium oxide or molybdenum oxide catalyst. As an example of the preferred process, a hydrocarbon feed, such as a naphthenic petroleum distillate, boiling within the range of 180 F. to 320 F., and obtained, for instance, by fractional distillation of a crude petroleum (from Kettleman Hills Oil Field in California) is passed at from about 900 F. to about 1200 F., desirably about 1000 F., over a vanadium oxide-alumina or molybdenum oxide-alumina catalyst. Space rate desirably is from 0.1 to about 2.0 volumes of liquid hydrocarbon feed per volume of catalyst per hour, and it is preferred to maintain a partial pressure of hydrogen in the reaction zone offrom about 30 to about 300 pounds per square inch.

The reaction product from such a hydroforming operation will contain not only the desired xylenes and additional aromatic hydrocarbon but also aliphatic hydrocarbons boiling over a wide range, including C4 and like materials. Initially, therefore, 1t is necessary to eliminate interfering hydrocarbons from this reaction mixture.

Preliminary conditioning of a :r1/Iene cui In order to obtain best results, it is important to select and properly condition a Ixylene cut from the overall aromatization reaction mixture by eliminating hydrocarbons which would interfere in subsequent process steps. In so doing, it is necessary not only to eliminate higheraind lower-boiling hydrocarbons but also to maintain the concentration of non-aromatic hydrocarbons boiling in the same range as the xylenes below certain maximum values. Thus, a preferred procedure involves:

(l) Eliminating gases (hydrogen and C4 or lower hydrocarbons) from the aromatization reaction mixture as by stripping in a fractionating column;

(2) Separating by fractional distillation hy* drocarbons boiling in the agsoline range and up to an end point of about 275 F. from the forescribed as a xylene fraction consisting essentially of ortho-, metaand para-xylenes. Since it will be apparent that the relative proportions of the isomeric xylenes in this purified fraction are substantially those formed and recovered under the equilibrium conditions reached in the hydroforming operation, this fraction also is designated hereinafter as an equilibrium mixture.

Separation of isomeric zylenes In addition to the foregoing factors it has been found that, in the separation of the equilibrium mixture of xylenes the order in which the xylene isomers are removed, as Well as the manner in which such separation' is accomplished, is of primary importance and that improved yields and purity may be obtained by a certain new combination of process steps. That is to say, it has been discovered that each stage in the separation process may be made to serve not only to recover an isomeric fraction from the equilibrium mixture of xylenes but also as a conditioning treatment for increasing the efciency in subsequent separations of isomeric fractions. Thus, it has been found not only that the boiling range and composition of the isomeric xylene equilibrium mixture from the hydroforming operation should be selected as previously described in order to condition this fraction for isomer separation but also that by first superfractionating the equilibrium mixture to initially separate higher boiling ortho-xylene in to 95% or higher purity, a lower boiling meta-para-xylene overhead fraction is obtained, which fraction is particularly adaptedand conditioned for selective removal of meta-xylene. This superfractio'nation is an important and difficult operation and in turn is followed by partial sulfonation to condition the xylene overhead for recovery of para-xylene.

Superfractionation of equilibrium xylene mixture Separation of ortho-xylene by superfractionation requires a highly eiilcient fractionating column; one equivalent to about thirty-five theoretical plates is necessary for practical operation, more desirably about forty-,five and preferably about sixty theoretical plates are utilized. Reflux ratios on distillate of from about 7 to 1 to about 12 to 1 llave been found satisfactory. Very close temperature regulation meta-xylene.

is important. but the distillation is so sensitive that control by temperature responsive devices has been found to give inefilcient though operable separation. A preferred method of superfractionation is to operate therfractionating unit continuously at a given constant feed rate while (1) removing overhead distillate and bottoms at a constant ratio corresponding to the feed rate and in `a relative proportion such that the desired purity of the ortho-xylene may be maintained, and (2) maintaining a constant volume of liquid in the still bottoms by controlling rate of heat input thereto. Maintenance of the constant volume of bottoms may be effected, for example, by a constant level control which increases the amount of steam admitted to the still heating unit when the level of the still bottoms begins to raise and decreases steam input when the volume of bottoms begins to drop below the pre-determined level. With the benefit of the foregoing instructions those skilled in the art of distillation will find no difllculty in effecting super-fractionation.

In conformance with this preferred superfractionation procedure, ortho-xylene of high purity may be obtained in good yield and the equilibrium mixture of xylenes is separated into a lowerboiling meta-para-xylene overhead fraction and a higher-boiling ortho-xylene fraction. Desir- ;bly the superfractionation is so controlled as to yield an ortho-xylene fraction of at least 90 ya purity. although purities as high as 97% or more have been obtained in good yield.

The meta-para-xylene overhead fraction from' this superfractionation stage is conditioned for removal of meta-xylene as has been previously indicated. In accordance with this invention, this lower-boiling overhead distillate next is conditioned for separation of a para-xylene fraction while simultaneously recovering a meta-xylene fraction, by partially sulfonating said overhead distillate as hereinafter described.

Partial sulfonation and selective hydrolysis To condition the lower-boiling distillate from the superfractionation stage for para-xylene recovery, said fraction is partially sulfonated to selectively form meta-xylene sulfonic acids and also an unsulfonated hydrocarbon mixture as an upper oil layer. This upper layerv is separated from the meta-xylene sulfonic acid layer (containing excess sulfuric acid) and is subsequently treated for recovery of a para-xylene fraction as will be described hereinafter.

The acid layer containing selectively sulfonated meta-xylene in the form of sulfonic acids is hydrolyzed and fractionally distilled to yield In general, the overall conditions of selective sulfonation and hydrolysis are as follows:

In the selective sulfonation it is preferred to use about two mols of sulfuric acid (96% to 98%) for each mol of meta-xylene in the mixture being treated, although these proportions are not critical. Weaker acid may be utilized down to about 75%, though more acid and higher temperature or greater time or both are required. The temperature of selective sulfonation ranges from about 80 F. to about 180 F. with a corresponding variation in the rate of sulionation. An upper limit of temperature is imposed by charring which occurs more readily with the more concentrated acids and at temperatures of 150 F. to 180 F. Presence of any parafln hydrocarbons in the distillate is a disadvantage at this stage, since paraiiins are more readily charred at the higher temperature and contaminate the unsulfonated product. The tim'e of contact between the hydrocarbon and the treating acid required for the partial sulfonation is a function of temperature, strength of acid and degree of mixing. Control of these variables to obtain partial and selective sulfonation in accordance with the foregoing principles will be well understood by those skilled in the art.

Upon completion of the selective sulfonation of the meta-xylene, stratification of the reaction mixture is effected (water may be added) and the meta-xylene sulfonic acid layer is submitted to selective hydrolysis. The rate of hydrolysis of the meta-xylene sulfonic acids is chiefly a function of the temperature. At atmospheric pressure, temperature and rate of hydrolysis therefore depend on the liquid boiling point of the aqueous acid solution. The boiling point of the solution, in turn, depends chiey upon the degree of dilution with water and approximates .the boiling point of sulfuric acid of similar water dilutions.

The meta-xylene sulfonic acid hydrolyzes more readily than the sulfonic acids of the other isomeric xylenes and by controlling temperature, meta-xylene may be recovered selectively from the sulfonic acid layer. Accordingly, the hydrolysis preferably is carried out by first heating the sulfonic acid mixture up to a temperature such that hydrocarbon vapors dlstill 0H below about 250 F. This initial distillate is condensed and comprises a para-xylene-rich fraction. Apparently it is obtained by freeing hydrocarbons more loosely held in the sulfonic acid solution as an emulsion or the like, since para-xylene sulfonic acid itself hydrolyzes less readily than the meta-xylene sulfonic acid.

The second stage of the hydrolysis operation is effected by heating the residual reaction mixture in the presence of water or steam at a temperature of from about 250 F. to about 300 F. Metaxylene vapors are thus selectively distilled ofi' from the reaction mixture at these temperatures. The meta-xylene vapors are condensed and collected. Meta-xylene fractions in 90% and higher purity are obtained.

Residual hydrocarbon sulfonic acids remaining in the still bottoms may bev hydrolyzed by raising the temperature of the last remaining reaction mixture to from 300 F. to 460 F., collecting, and condensing hydrocarbon vapors therefrom to form a second para-rich xylene fraction.

This para-xylene-rich fraction together with that distilled initially from the sulfonic acid solution, may be utilized as desired but preferably are added to the unsulfonated hydrocarbon mixture formed in the partial sulfonation stage and separated from the acid layer. This composite unsulfonated hydrocarbon mixture is next treated for separation recovery of para-xylene.

Para-xylene recovery Each of the foregoing major process steps has served as a conditioning treatment for facilitating the maximum recovery of paraxylene in high purity and economical yields. The importance of these previous process steps as a conditioning treatment is illustrated in Figure 2 of the drawing. where the effect of the ratio of orthoand meta-xylene on the maximum recovery of para-xylene of any given degree of purity is illustrated. From Figure 2 it becomes apparent that by controlling and interrelating the removal of orthoand meta-xylenes from the equilibrium xylene mixtures so that the ratio of orthoto meta-xylene is approximately 1 to 2, a maximum recovery of paraxylene is obtained for any degree of contamination with these two isomers. Thus, it is preferred to control and correlate the superfractionation and the selective partial sulfonationv operations so that the ratio of orthoto metaxylene in the para-rich fraction ranges from about 25 parts ortho to 75 parts meta, on the one hand. to about 40 parts ortho to 60 parts meta, on the other hand, with approximately 33 parts ortho to 66 parts meta being optimum. The foregoing ratios refer only to the relative proportions of orthoto meta-xylenes in the three-component mixture. It will also be observed from Figure 2 that the percentage of para-xylene recoverable for high purities drops rapidly as the content of combined orthoand meta-xylenes exceed '10%. Preferably the superfractionation and selective sulfonation operations are controlled so as to maintain not only the foregoing ratios of orthoto meta-xylenes but also to reduce the total content of these isomers to less than 60%, desirably no greater than 50%, and preferably less than about 45%, while retaining a total of about 10% or more of these two isomers in the feed to the paraxylene recovery stage.

The para-xylene is recovered from the unsulfonated hydrocarbon mixture and separated after the selective sulfonation stage by partially freezing said mixture in a low-temperature crystallization stage to selectively crystallize para-xylene. The temperature in this first crystallization stage should be reduced to at least -50 F. and preferably to about 65 F. When the isomeric xylene fraction is free of other hydrocarbons, temperatures below 67 F. serve to contaminate the crystallized para-xylene fraction with other isomers as impurities. However, for each 10% of other hydrocarbons boiling in the same range as the xylenes each of the foregoing crystallization temperatures should be lowered about 10 F.

After bringing the hydrocarbon mixture down to the foregoing temperatures and retaining the mixture at said temperature until crystal formation has occurred, the resulting crystals are separated from the mother liquor. vFor additional purification the separated crystals are then solvent washed, centrifuged or melted and subjected to a second partial freezing operation.

In a second crystallization stage a higher temperature is utilized. This higher temperature is at least about 50 F. above that of the lower stage and more desirably about 80 F. thereabove. Optimum temperature ranges for the second stage crystallization are also influenced by the purity of the product and the yield desired. In general,v a temperature below at least 40 F. and above about F. should be utilized. A more desirable temperature range is from about 15 F. to 30 F. Each of these crystallization temperatures should be lowered about F. for each 10% other hydrocarbon component as stated in connection with the low temperature crystallization stage.

After holding the hydrocarbon mixture at the 70 specified temperature until crystal formation occurs in the second stage, the resulting solid para-xylene fraction is separated from the remaining liquor and passed to storage. Upon re- 10 tion stage, good overall yields of para-xylene in relatively high purity are obtained.

In order to guide those skilled in the art in the practice of this invention, details of one suitable method have been illustrated diagrammatically in Figure 1 of the drawing wherein:

A hydroform'ed non-aromatic petroleum stock is fractionated as previously described to yield a xylene fraction consisting essentially of an equilibrium mixture of the isomeric xylenes. The last stage of this preliminary fractionation and conditioning treatment as here shown comprises a fractionating column I0 to which hydroformed xylenes are fed by way of inlet line II. and the equilibrium mixture of isomeric xylenes is distilled overhead by way of outlet I2 through condenser I3 to the superfractionation stage of the process.

The equilibrium mixture of isomeric xylenes fed to the superiractionator may vary somewhat in composition, the following being exemplary:

Per cent Meta-xylene 46 Ortho-xylene 28 Para-xylene 21 Ethyl benzene 5 or less This equilibrium mixtureA of isomeric xylenes enters superfractionator Il by way of inlet line I5 and is separated into a lower-boiling metapara-xylene overhead fraction and a higherboiling ortho-xylene fraction simultaneously to condition the overhead for recovery of metaxylene and to yield ortho-xylene in high purity. The control and operation of the superfractionator has been described hereinbefore and will not be repeated. Suillce to say that ortho-xylene purities as high as 97% have been obtained without dimculty in good yield, although a to ortho-xylene fraction is usually preferred. For certain uses 85% ortho-xylene is acceptable.

As here shown, the higher-boiling ortho-xylene fraction passes from superfractionator I4 by way of outlet line I6 through cooler I1 to orthoxylene storage I8. The lower-boiling meta-paraxylene overhead fraction flows through outlet.. conduit I9 and condenser 20 to a suitable surge tank or the like 2|.

This meta-para-xylene yfraction has been conditioned for recovery of meta-xylene by the previous superfractionation treatment. The composition of this fraction which is next fed to the meta-xylene recovery stage may vary within limits previously disclosed. An analysis of illustrative fraction is:

Per cent Meta-xylene 56 Para-xylene 25 Ortho-xylene 13 Ethyl benzene 6or less To recover metal-xylene the feed stock is partially sulfonated as indicated at 22 to form a sulfonic acid layer and an oil layer which are separated by decantation or any other suitable method as shown at 23.

An illustrative operation for the partial sulfonation and selective formation of meta-xylene sulfonic acids comprises contacting the hydrocarbon feed by vigorous agitation with 96% sulfuric acid in an amount equivalent to two mois of the acid per mol of meta-xylene in the' feed. The reaction mixture may be held at F. as acid is added and then raised to about F. for apcycling the residual. liquor to the first crystalliza- 75 proximately two hours.

11 The reaction mixture may be diluted with water and the sulfonic acid layer is separated lat 2l and' then passed to the selective hydrolysis stage of the process. As here shown, the sulfonic acids dissolved in the excess sulfuric acid are fed to still 24 and may be further diluted with water introduced by way of. valve-controlled line 2S. The reaction mixture then is heated to a temperature of 250 F. while distilling oil an initial hydrocarbon fraction which flows by way of line 21 and condenser 28 to storage 20. An exemplary composition of this' initial overhead and pararich fraction is:

Per cent 47 The remaining reaction mixture (first still bottoms) passes to a second still Il as indicated and additional water or steam may be introduced by way of valve-controlled line 32 as shown for eecting selective hydrolysis of meta-xylene. The temperature of the reaction mixture maintained in this second stage should be from about 250 F. to about 300 F. Meta-xylene sulfonic acids are selectively hydrolyzed under these conditions, and at atmospheric pressure the resulting meta-xylene is distilled off as an overhead fraction through outlet line 33 and fio'ws through condenser I4 to meta-xylene storage 36. Metaxylene fractions of varying purity may be obtained in this selective hydrolysis operation depending primarily upon the care taken in control ofthe partial sulfonation reaction conditions and on the temperature and time of hydrolysis and distillation of the acid layer. An exemplary composition of a meta-xylene fraction is' Per cent 'Meta-mene 9o Ortho-xylene Para-xylene and may be stored at 29 with the overhead fracltion from theilrst hydrolysis still to form a xylene fraction rich in the para isomer. An exemplary composition of the overhead from this third stage of the hydrolysis is:

Per cent Para-xylene 65 Meta-xylene 16 Ortho-xylene 13 Ethyl benzene 6or less Para-xylene 43 Meta-xylene Ortho-xylene Ethyl benzene 12 76 It will be observed that the ratio of orthoto meta-xylene in the above fraction is approximately l to 2 and therefore is substantially at the optimum value for maximum recovery of para-xylene in best purity. Some additional ad- Justment in this ratio may be made by blending the xylene fraction from intermediate storage 28 as indicated by flow line 43. It will be noted that each of the fractions making up this blended stock contained a higher ratio of orthoto metaxylene than 1 to 2 and may accordingly be utilized to partially adjust the slightly lower ratio in the unsulfonated oil stored at 42.

The feed to the para-xylene recovery system ilows from storage 42 by way of mixer 44 to a, iirst low-temperature crystallization stage 46. The preferred ranges of temperatures and other conditions for partially freezing the feed to form a solid para-xylene fraction were discussed hereinbefore.4 An exemplary operation comprises cooling the feed to approximately 77 F. and holding the same at this temperature until partial solidication of the hydrocarbon mixture occurs to selectively separate a solid or crystalline paraxylene fraction.- Normally. satisfactory solidiflcation or crystal formation is obtained in approximately ten minutes. 'Ihe mother liquor is separated from the solidified para-xylene frac Per cent Meta-xylene 48 Ortho-xylene 22 Para-xylene l2 Ethyl benzene 18 It will be found that upon continuous recycling of the mother liquor to the superfractionating unit, the ethyl benzene concentration builds up in the system where it is contained in the feed. When and if this occurs, excess ethyl benzene may be eliminated from the rst stage mother liquor prior to recycling the same. Any suitable method, such as superfractionation. known to those skilled in the art may be used for reducing the ethyl benzene content of the mother liquor. This may be done in the zone 49, designated Mother liquor" in Figure 1 by superfractional distillation means such as described in connection with zone I4. 4

The para-xylene fraction separated by selective crystallization in the lower-temperature stage is of good purity: for example, from to a typical composition analyzing 88% para-xylene. However, in the preferred embodiment of the invention the melted crystals are fed to a higher-temperature crystallization zone and a para-xylene fraction again selectively separated as indicated at l2. This higher-temperature crystallization and the conditions therefor have been described previously in detail, so that the general conditions for operation need not be repeated. By way of example, however, by cooling the melted crystals in the second crystallization stage to a temperature of about +20 F., 96% para-xylene is obtained, and by recycling the mother liquor to the low-temperature crystallization stage, para-xylene in this or higher purity may be obtained in the yields approximating those indicated in Figure 2 of the drawing. This recycle of mother liquor is effected after separation of the para-xylene crystals at 53 as shown in Figure 1. The separated crystals pass to paraxylene storage 54, and the mother liquor flows by way of line 55 to mixer 4l. An exemplary composition of the second stage mother liquor recycled to the iirst stage crystallization is illustrated by the following analysis:

Per cent Para-xylene 65 Meta-xylene 19 -Ethyl benzene 9 Ortho-xylene 7 The foregoing process is capable of giving essentially 100% yields, based on the original feed of the invention are applicable to the separation of other dialkyl benzene isomers from mixtures thereof; for example, mixtures of ortho. meta and para, isomers of diethyl benzenes, mixtures of ortho, meta and para isomers of dipropyl benzenes, mixtures of ortho, meta and para isomers of ethyl methyl benzenes, mixture of ortho, meta and para isomers of ethyl propyl benzenes, and the like. Accordingly, the invention is not to be limited by the examples except as defined in the appended claims.

We claim:

1. A process for separating ortho-xylene, metaxylene and para-xylene into individual fractions each predominantly a single xylene from an equilibrium mixture of such xylenes which comprises superfractionating the mixture of such xylenes to separate an ortho-xylene fraction and to form a predominately metaand para-xylene overhead fraction, partially sulfonating said overhead fraction to selectively convert a major portion of the meta-xylene into its sulfonic acid, separating from said sulfonation reaction mixture an unsulfonated reaction mixture containing ortho, metaand para-xylenes but predominantly p-xylene, hydrolyzing and distilling said meta-xylene sulfonic acid to obtain a metaxylene overhead and a residue of orthoand paraxylene, sulfonic acids, hydrolyzing them and combining the resulting orthoand para-xylenes with said unsulfonated reaction mixture which has been separated from said sulfonation reaction in such manner as to form a xylene mixture containing about one part of ortho-xylene to each two to three parts of meta-xylene with the combined orthoand meta-xylene content of the mixture being from about 10% to about 50%, partially freezing said last mentioned xylene mixture to form a solid para-xylene fraction and a mother liquor, separating said solidified paraxylene fraction from said mother liquor, and returning said mother liquor to the superfractionating step of the process.

2. A process for separating ortho-xylene and para-xylene into individual fractions each predominantly a single xylene from an equilibrium mixture of ortho, meta, and para-xylenes which comprises superfractionating such equilibrium mixture to separate an ortho-xylene fraction containing at least 90% ortho-xylene and to form a predominantly metaand para-xylene overhead fraction containing at least 50% meta-xylene, partially sulfonating said overhead fraction to selectively convert a major portion of the metaxylene into its sulfonic acid, separating from said sulfonation reaction mixture an unsulfonated hydrocarbon mixture containing ortho, meta, and para-xylenes but predominantly p-xylene, and with a ratio of about one part ortho-xylene for each two parts meta-xylene and with the combined orthoand meta-xylenes content of the separated unsulfonated hydrocarbon mixture being from about 10% to about 50%, partially freezing said unsulfonated hydrocarbon mixture to form a solid para-xylene fraction and a mother liquor, separating said para-xylene fraction from said mother liquor, and returning said mother liquor to the superfractionating step of the process.

3. A process for separating a mixture of o, m, and p-xylene into individual fractions of o, m, and p-xylene each of at least about 90% purity, which comprises charging an equilibrium mixture of said xylenes to a superfractional distillation zone, removing and recovering said o-xylene fraction therefrom, removing a m, and p-xylene rich vapor therefrom, condensing the vapor, selectively sulfonating the major portion of the m-xylene in the condensate, diluting the sulfonation reaction mixture with water, separating the unsulfonated layer containing at least 40% p-xylene and neutralizing it, passing the acid phase from the last named separation step to a distillation zone, steam distilling it at a temperature below which substantial hydrolysis of the xylene sulfonates occurs, removing and recovering the unsulfonated p-xylene rich steam distillate, selectively hydrolyzing the m-xylene sulfonate in a first hydrolyzing zone, recovering said m-xylene fraction, selectively hydrolyzing the oand p-xylene sulfonates in a second hydrolyzing zone, combining the o-and p-xylene rich distillate obtained therefrom with the pxylene rich steam distillate, then combining at least a portion of the mixture with said neutralized unsulfonated oil to adjust the ratio of oand m-xylenes in the range of about 25 parts o-xylene to parts m-xylene to about40 parts o-xylene to 60 parts m-xylene,rand in such proportions that the resulting mixture contains at least 40% of p-xylene, fractionally crystallizing the last named mixture, separating the mother liquor and recycling at least a part thereof to the superfractional distillation step, melting the crystals of predominantly p-xylene obtained from said crystallization step, recrystallizing it at a higher temperature in a second crystallization step, recovering the said p-xylene fraction therefrom and recycling the mother liquor from the second crystallization step to the first crystallization step.

4. A process for separating a mixture of o-, m, and p-xylene into individual fractions of o, m, and p-xylene each of at least about purity which comprises charging an equilibrium mixture of said xylenes to a superfractional distillation zone, removing and recovering said o-xylene fraction therefrom, removing a mand p-xylene rich vapor therefrom, condensing the vapor, selecively sulfonating the major portion of the mxylene in the condensate, diluting the sulfonation reaction'mixture with water, separating the unsulfonated layer containing at least 40% lli-xylene and neutralizing it. passing the acid phase from the separation step to a distillation zone, steam distilling it at a temperature below which substantial hydrolysis of xylene sulfonates occurs, removing and recovering a p-xylene rich steam distillate therefrom, selectively hydrolyzing the m-xylene sulfonate in a ilrst hydrolyzing zone, recovering said m-xylene fraction, selectively hydrolyzing the o-and p-xylene sulfonates in a second hydrolyzing zone, combining the o, and pxylene rich distillate obtained therefrom with the p-xylene rich'steam distillate, then combining at least a portion of the last named mixture with said neutralized unsulfonated portion to adjust the ratio of oand m-xylenes in the range of about 25 parts o-xylene to 70 parts m-x'ylene to about 40 parts o-xylene to 60 parts m-xylene, and in such proportions that the resulting mixture contains at least 40% of p-xylene, fractionally crystalllzing the mixture in a first step at a temperature not less than about 50 F. nor more than about 67 F. when using substantially pure xylenes, lowering the temperature limits about C. for each 10% of non-xylene hydrocarbons in the mixture, separating the mother liquor and recycling at least a part thereof to the superfractional distillation step, melting the crystals of predominantly p-xylene obtained from said first crystallization step, recrystallizing it at a higher temperature in a second crystallization step, recovering the said p-xylene fraction therefrom, and recycling the mother liquor from the Asecond crystallization step to the first crystalis sulfonated, separating the unsulfonated layer containing at least 50% D-xylene and neutralizing it, passing the acid phase from the last named ture of o, mand p-xylene into individual fracseparation step to a distillationv zone, distilling the unsulfonated hydrocarbons therefrom at a temperature below which substantial hydrolysis of the sulfonates occurs, recovering a p-xylene rich distillate, selectively hydrolyzing the mxylene sulfonate in a first hydrolyzing zone, recovering said m-xylene fraction, selectively hydrolyzing the oand p-xylene sulfonates in a second hydrolyzing zone, combining the oand p-xylene obtained therefrom with the p-xylene rich distillate and then combining at least a portion of the mixture so obtained with said unsulfonated layer to adjust the ratio of ortho to meta xylenes to within the range of from about parts o-xylene to 75 parts m-xylene to about parts o-xylene to about 60 parts m-xylene and in such proportions that the resulting mixture contains at least 40% of pxylene, thereafter fractionally crystallizing the mixture in a first crystallization step, separating the mother liquor and recycling at least a portion of it to the superfractionai step, melting the crystals of predominantly p-xylene, recrystallizing said p-xylene at a higher temperature in a tionsof o, mand p-xylene each of at least about purity which comprises charging an ,equilibrium mixture of said xylenes to a super fractional distillation zone. removing and recovering said o-xylene fraction therefrom, removing vapor rich in mand p-xylenes from said zone, condensing the vapor, subjecting the condensate to selective sulfonation under conditions whereby the major portions of m-xylene therein is sulfonated; separating the unsulfonated layer containing at least 50% p-xylene, passing thehacid phase from the separation step to a distillation zone, distilling the unsulfonated hydrocarbons `least a portion of the mixture so obtained with said unsulfonated layer to adjust the ratio of ortho to meta xylenes to within the range of from about 25 parts o-xylene to 75 parts mxylene to about 40 parts o-l-xylene to about 60 parts m-xylene and in such proportions that the resulting mixture contains at least 40% of pxylene, thereafter fractionaily crystallizing the mixture, separating the mother liquor-,removing at least a part of the hydrocarbons other than xylenes and comprising ethyl benzene from the mother liquor by fractional distillation and recycling the resultant xylene containing product to the super fractionation zone, melting the crystals of predominantly p-xylene, recrystallizing p-xylene inay second crystallization at a higher temperature than in the first recrystallization step, recovering said D-Xylene fraction therefrom, and recycling the mother liquor from the second crystallization step to the flrst crystallization step.

7. A continuous process for separating a mixture of o, mand p-xylenes into individual fractions consisting principally of o, mand pxylene, each fraction being of at least about 90% vpurity which Ycomprises charging an equilibrium mixture of o, mand p-xylene to a superfractional distillation zone, removing and recovering said o-xylene fraction therefrom, removing a mand p-xylene rich vapor therefrom, condensing the vapor, selectively sulfonating the major portion oi the m-xylene contained in the condensate, diluting the acid layer of the sulfonation reaction mixture with water, separating the unsulfonated layer containing at least about 50% of p-xylene and neutralizing it, passing the acid layer containing the xylene sulfonate from the separation step to a distillation zone, distilling unsulfonated hydrocarbons therefrom ata temperature below which substantial hydrolysis of the sulfonated xylenes occurs, recovering a pxylene rich distillate from said distillation step, selectively hydrolyzing the major portion of the m-xylene in a first hydrolyzing zone, recovering said m-xylene fraction, selectively hydrolyzing the oand psulfonates remaining in said acid layer in a second hydrolyzing zone, combining at 17 least g portion of the mixture of oand p-xylenes so obtained with said unsulfonated oil to' adjust the oand m-xylene ratio within the range o; about 25 parts of o-xylene to 70 parts m-xylene to 40 parts o-xylene and not more than 60 parts m-xylene. in such proportions that the resulting mixture contains at least 40% p-Xylene therein, fractlonally crystallizing the p-xylene in said mixture, separating the mother liquor, recycling at least a portion of it to the superfractionation step, melting the crystals of p-xylene, recrystalllzing them at a higher temperature than the said rst crystallization step, recovering the said p-xylene traction therefrom and recycling the mother liquor from the second crystallization 15 ltep to the first crystallization step.

HOWARD P. HETZNER. ROBERT J. MILLER.

` REFERENCES CITED The following references are of record in the le of this patent: i

UNITED STATES PATENTS Number Name Date 1,940,065 Spannagel et ai. Dec. 19, 1933 2,393,888 Cole Jan. 29, 1946 2,398,526 Greenburg Apr. 16, 1946 OTHER REFERENCES 

1. A PROCESS FOR SEPARATING ORTHO-EXYLENE, METAXYLENE AND PARA-XYLENE INTO INDIVIDUAL FRACTIONS EACH PREDOMINANTLY A SINGLE XYLENE FROM AN EQUILIBRIUM MIXTURE OF SUCH XYLENES WHICH COMPRISES SUPERFRACTIONATING THE MIXTURE OF SUCH XYLENE TO SEPARATE AN ORTHO-XYLENE FRACTION AND TO FORM A PREDOMINATELY META- AND PARA-XYLENE OVERHEAD FRACTION, PARTIALLY SULFONATING SAID OVERHEAD FRACTION TO SELECTIVELY CONVERT A MAJOR PORTION OF THE META-XYLENE INTO ITS SULFONIC ACID, SEPARATING FROM SAID SULFONATION REACTION MIXTURE AN UNSULFONATED REACTION MIXTURE CONTAINING ORTHO-, META- AND PARA-XYLENE BUT PREDOMINANTLY P-XYLENE, HYDROLYZING AND DISTILLING SAID META-XYLENE SULFONIC ACID TO OBTAIN A METAXYLENE OVERHEAD AND A RESIDUE TO ORTHO- AND PARAXYLENE, SULFONIC ACIDS, HYDROLYZING THEM AND COMBINING THE RESULTING ORTHO-AND PARA-XYLENES WITH SAID UNSULFONATED REACTION MIXTURE WHICH HAS BEEN SEPARATED FROM SAID SULFONATION REACTION IN SUCH MANNER AS TO FORM A XYLENE MIXTURE CONTAINING ABOUT ONE PART OF ORTHO-XYLENE TO EACH TWO TO THREE PARTS OF META-XYLENE WITH THE COM- 